Or, better known as "ISBNs". It's a way to uniquely identify each book out there, whether a dead-tree book or an eBook. Many places require an ISBN before they'll make your masterwork available for sale, so.. I'm stuck getting some (one per book).
Now, these are NOT free! The most affordable way is to hit Bowker and buy a block of 10 for $250. Use your credit card as they don't take Paypal! Wave goodbye to your money and hello to 10 long numbers.
If you're a publisher, you'll burn through 10 pretty quickly. For a self-publishing author, 10 is a decent sized pile! If I somehow write 2 books a year, then my 10 will last a long time - and this is assuming I can crank out 2 a year! I suppose it's possible.. maybe.
My estimate is: one for "Dead Ringer", one for "Threshold", one for "Strip", one for an as-yet-un-named steampunk erotic horror novel, 2 more if I finish the Threshold trilogy. That leaves me 4 to play with, say 1 for a Raven Delacroix novel in New Orleans, and 1 if I turn The Vicky Chronicles into a collection. The last 2 are in case I do a sequel or prequel to "Dead Ringer" (or both).
Right there is about six to seven years of HARD LABOR. So, $250 over that much time isn't too bad. It's going to cost more for cover art per-book than the ISBN. Ugh.
Apropos of nothing: I'm trying to figure out how to link updates from my blogs to my main Website so that you can just visit Woolyloach.com and see all the latest blog posts. I think RSS will do it, but it's going to take some work.